Project Summary/Abstract Reverse shoulder arthroplasty (RSA) is a surgical procedure most often performed on elderly patients with symptomatic shoulder arthritis who are unable to elevate their arm due to an irreparable rotator cuff tear. Prior to surgery, patients may have pseudoparalysis that severely limits their ability to independently perform common activities of daily living (ADLs). RSA restores strength and range of motion (ROM) by allowing the deltoid to compensate for an absent rotator cuff. Clinical outcomes after RSA are generally favorable, however, complication rates remain high and functional outcomes (e.g. range of motion and strength) remain suboptimal. Research on cadavers and computer simulations suggest that functional outcomes may be improved by modifying the design and placement of the shoulder prosthesis. However, controversy still exists as to the best implant geometry and implantation technique for maximizing shoulder function after RSA. This controversy persists, in large part, due to the paucity of quantitative in vivo motion data to guide this debate. Therefore, a critical need exists for quantitative in vivo shoulder motion in RSA patients. The long-term goal of our research is to improve clinical and functional outcomes for patients who receive RSA. The goals of this project are: 1) to identify the effects of prosthesis design and placement on shoulder motion and strength after RSA, and 2) to identify the motion and strength parameters that are associated with improved clinical outcomes. Participants will be 30 RSA patients who are 1 to 5 years post-surgery. We will use biplane radiography to image shoulder motion while participants perform scapular plane abduction, internal/external rotation at 90? abduction, touching the back of the head, and touching the middle of the lower back. A validated tracking system that has a precision of better than 1 mm in translation and 1? in rotation will determine shoulder joint motion during each movement. Shoulder strength will be measured using isokinetic dynamometry during flexion/extension, internal/external rotation, and ab/adduction. The Specific Aims are to determine the in vivo effect of prosthesis placement and design on shoulder kinematics and strength after RSA, and to identify the shoulder kinematics and strength parameters that are associated with improved clinical outcomes. As part of a secondary analysis, we will determine the size and location of the contact region between the glenosphere and humerosocket to characterize the mechanical environment of the prosthesis, and the distance between the humerosocket and scapula to identify in vivo motions that may lead to scapular notching. The expected outcome of this study will be novel data relating prosthesis placement and design to in vivo shoulder function and clinical outcomes after RSA. This knowledge can be used to improve clinical and functional outcomes, leading to improved quality of life for RSA patients. This study will provide preliminary data to inform future studies to investigate the effects of age, comorbidities, and rehabilitation on shoulder kinematics, strength and clinical outcomes after RSA.